Some systems for counting, denominating, and for sorting documents such as paper currency require documents to be separated so that documents can be fed one by one along a transport path. If, however, documents get stuck to neighboring documents in a stack, it can become difficult or impossible to accurately count and/or sort individual documents.
A known method for determining whether a device is transporting a single document or doubled documents is to pass light through the document(s). Because doubled documents will generally not allow as much light through as single documents, a light sensor on the opposite side of the document from a light source will give a lower reading when doubled documents are encountered. This does not completely solve the problem, however. Because of the variations of cleanliness or dirtiness in individual bills, the transmitted light reading may be inaccurate. This is especially true in countries where the quality of circulated currency varies greatly. For example, an especially dirty bill may pass relatively little light through, thus appearing as two bills to the light sensor. Conversely, doubled bills that are especially clean or worn may let more light through than an average bill, giving the sensor the impression that a single bill is being transported. Thus, there exists a need for an system for detecting doubled bills that accounts for differences in the qualities of individual bills.